Wave guide with dual purpose gas discharge device



1956 GOLDSTEIN ET AL 2,773,243

WAVE. GUIDE WITH DUAL PURPOSE GAS DISCHARGE DEVICE Filed July 25, 1952 BIAS vaL TAGE SWITCH SDI/RC5 INVENTORS LAD/.SLAS GOLDSTE/N HERBLRT F. ENGELMAIWV WAVE GUIDE WITH DUAL PURPOSE GAS DISCHARGE DEVICE Ladislas Goldstein, Urbana, Ill., and Herbert F. Engelmann, Mountain Lakes, N. J., assignors to International Telephone and Telegraph Corporation, a corporation of Maryland Application July 25, 1952, Serial No. 300,998

3 Claims. (Cl. 333-43) This invention relates to a gas discharge device for use at ultra-high frequencies and more particularly to a gas discharge device which performs the dual function of a T-R box and attenuating switch. By the abbreviation T-R as used, for example, in T-R box, I refer to the wellknown abbreviation for the term transmission-reception. This will be the meaning of this abbreviation where used in the specification and claims.

In order to be able to control the flow of guided electromagnetic energy, a variation in circuit elements must be produced in the guiding system. Such variations can be achieved mechanically, 'but in general the time duration involved in mechanical operations is very limited, and if a very rapid control of the energy flow is desired, other means must be provided.

The controlling circuit elements must have adequate electrical conductivity at the frequency of the low or high intensity microwave fields to be controlled. The ideal inertialess controlling circuit element for micro wave energy is a pure electron gas created in an appropriate volume of the waveguide system. It is known from the free electron model of the electrical conductivity in metals that conductivity is proportional to the density of the electrons responsible for it. For this reason to obtain high electrical conductivities in an otherwise non-conducting medium by the creation of an elec tron gas in that medium, the density of such a gas must be very high. It is also known that because of the space charge forces which arise in a pure electron gas, only limited densities can be obtained. It is necessary that the controlling circuit element constituted by an electron gas have the required minimum density for reasonable conductivities, and this can only be obtained if the electron gas is space charge compensated. A simple way to produce a high electron density, space charge compensated, free electron gas is to produce a discharge in a non-electronegative gaseous medium. There, under certain conditions, space charge compensation occurs, the positive ions being produced with the electrons in equal numbers. Under these conditions, however, the electron gas is not a pure gas but has certain partial pressures in the gas mixture in which it is produced.

T-R box operation and control of low power, high frequency, energy flow in switching operations are both dependent on the complex conductivity of a non-pure, space charge compensated electron gas. The specific requirements, however, are different in the two operations.

As the main purpose of a T-R tube is to connect the antenna to the transmitter While effectively protecting the receiver from possible damage due to the high power, radio-frequency pulse of the transmitter, the T-R tube is generally operated by the leading edge of high power, radio-frequency pulse of short duration (less than 1 microsecond). This device operates, therefore, at high power levels. It must perform its function in a very nited States Patent "ice short time and recover during the shortest possible time after the cessation of the transmitter radio-frequency pulse.

The discharge in the gas of a T-R cavity is produced by the dissipation of a fraction of the energy of the transmitter power pulse. In a T-R tube a gas or gas mixture is needed in which a discharge will be started with the lowest possible radio-frequency field at the operating frequency, so as to minimize spike energy transmitted through the T-R tube to the receiver before a discharge takes place. Furthermore, in order to obtain short recovery times, the free electron removal from the radiofrequency gap is obtained through electron attachment by electronegative components of the gas mixture used. In order to obtain very rapid recovery, the electronegative gas vapor in the T-R tube should have fairly high pressure, which gives to the whole gas mixture a strong electronegative character. In order to build up very rapidly the electron density necessary to detune 'the cavity sufiiciently by means of the radio-frequency pulse, the mean free path of the electrons should be relatively large so that they can be accelerated to inelastic collisional levels in the gas in a mean free path. This requires a relatively low total gas pressure (5 to 15 mm. mercury).

An attenuator switch for radio-frequency energy flow,

'as opposed to the TR function, should attenuate low power radio-frequency signal echoes during predetermined time, relatively longer than the transmitter pulse (several hundred microseconds or longer). The amplitude of the radio-frequency field is small and entirely insuflicient to maintain an adequate electron density in the radio-frequency gap. It is not possible to operate the discharge by the radio-frequency power of the transmitter either. An appropriate electron density, whose magnitude depends on the attenuation required and can be easily calculated, is produced and/or maintained in the switching tube by external means. One requirement of a good switching device is that the attenuation must be mostly reactive. No large dissipation of the low power radio-frequency energy in the gas can be tolerated. In view of the fact that high free electron density for long durations is needed which can he obtained and maintained with low power sources (direct-current or the low frequency alternating-current), the electron removal process in such a switch should be minimized. In particular no electronegative gases or mixtures thereof should be utilized for this purpose.

It is seen that the nature of the gases in T-R tubes and switching tubes as defined above are very different. However, we have found that this difference in the op timum operation of a T-R tube and a switching tube does not exclude the possibility of using the same gas mixture in a dual purpose tube, provided appropriate electron densities can be readily produced in an electronegative gas mixture used in T-R tube geometries. In view of the required short time in T-R action, the gas mixture should have an electronegative character; means must therefore be provided to produce a gas discharge plasma areformedby a.pairiofrtruncatedsconmshapednelectrodes,

one of which is hollow. The cones are located with their slightly truncated apex ends opposed and in alignment.

insulated. therefrom. The tubet'is coupledninzawaveguide system and functions in 'a:- manner to ;be.described =hereinafter.

The above-mentioned .andother .features and'objects of this invention will become :more: apparent 2 by reference to;the following,descriptiontakenr in: conjunctionzwith the accompanying: drawing; in .;-which The figure-of the, illustrationiis ia apartial 'crossrsectional view of .one embodiment of atgasidischarge; device; of i this Contained within the chamberare a pair oficone-shaped electrodes 3 and darranged with their slightly truucated. apex ends in alignment and slightlyseparated to form a. capacitance gap 5. The-cone:4-is adjustable along its'axis to vary the frequencyiat-which the,resonatorfunctions.

The cone 4 is connected to an adjusting screw device;6'=

capableof operation by meansofa screwdriver'placed in slot 7; A flexiblehermetic seal is-maintained between the electrode 4 and Waveguide wallS-by means of a thin; diaphragm, 9 sealed to a shoulder 9a on the waveguide,

wall 8 along its peripheral edge and sealed to thezelectrode 4 at its center. The cone-shaped electrode 3 is hollow having an axialopening l1v extending therethroughflandi emerging at the small end of the electrode -3 communicat:

ing with the chamber 2. Contained within theopening 11. is a rod of uniform diameter-functioning as'fkeep-alive. electrode 12. The keep-alive-electrode- 12.is insulated from the cone-shaped electrode 3.: by; a sheath 13 constructed of an insulated material, such as glass, and may be coupled to a source of electric 'currentl0ithrough lead 14. The cone 3 is inserted into the .space ;charge-chamber.

2 through a radio-frequency. choke Hand. is electrically isolated from, the remainder: of the: transmission system.

by insulating walls -16 composedxof any suitable: mater1al, such as-glass..

The resonant cavity 2; is :provided, with walls- 17 i'and 18' composed of an appropriate"substanceewhiehpermits the free passage of electromagnetic, energy t therethrough and is sealed in a gas tight union witlr thewalls of :waveguide- 1. An appropriatemixtureof gases ,is introduced into the space charge chamber 2 by means: well-known toe-those skilled in the art. We have found that a suitable gas mixture comprises argon at 10. mm. mercury pressure and water at 6 millimeters mercury pressure.

The T-R tube functions of thedevice of this inventionciently increased that a gaseous discharge occurs, and as a result the resonant chamberZ is detuned for the operating frequency, whereby the outputside of the waveguide. is disconnected from thesystem, At the conclusion of they pulse conduction, the resonator-again;becomes-tuned for the operating frequency. The operation ,of'thedeviceasa.

T R box depends upon. thefact thatagaseous :dischargc will'occur at a predetermined small potential above the The hollow" coneris insulated-from the; remainder:- of: the structure and'z'hasa: keep-alive 761Ct0d6 einserted: in the hollow portion coaxial with said hollow conezanddirect current biaspotential applied, to the keep-aliveelcc: trode 12.

In order to function as an attenuating switch device for low peak voltages, the insulated cone-shaped electrode 3 is connected through an appropriate switch 19 to a source of electric energy 20 so thatwhen switch 19 is closed, a discharge will occurswhich detunesthe resonant cavity 2. When the resonant cavity 2;is Vdetuned, the output side of the waveguide transmissionsystem 1 will be isolated from any radio=frequency energycoupled to the input side; We have found that this gas-tube. switch, when supplied with a striking potential of approximately 400 volts resulting in a current of 0.75 milliamperes will provide 35 decibels of attenuation. T he: gas tube-discharge was "pulsed at a l kilocycle rate of a- 50 percentduty cycie.

While we have describ'ed'above the principles of our invention in connection with specific apparatus, it is to be clearly understoodithatsthisi description is madev only by way of example andsnotas'alimitation to the, scope of ourinvention as set forth :in theobjects-thereof and in the accompanyingclaims.

We claim: 1.- A.combinationiT=R;and attenuating switch gas discharge devicesystem providing selective T-R action and attenuation: of; low-power radio-frequency energy comprising means defining a resonant chamber, an ionizable gasfilling said.chamber, first and second electrodes disposed withinsaidchamber and insulated from one another'fordirect currents and electrically connected .to the means defining a-resonant chamber at radio frequency, means-to couple a-source of biasingvoltage between said firstand second electrodeswherebysaidelectrodes serve as keep-alive electrodes, and a third electrode spaced from said first andsecond electrodes and insulated therefrom, said first and third electrodes being so disposed that highpower radio-frequency; energy propagating through said chamber will cause ionization of'said gas and a discharge directly between said first and third electrodes, low-power radio-frequency energy being insutficient of itself to cause saiddischarge, asource of voltage, and switching means operable independently of high-power radio-frequency energy-propagating through said chamber to couple said source of voltage between said first and third electrodes whereby during the, on period of operation of said switching means low-power.radio-frequency energy propagating through said chamberiwill cause a discharge directly between said first ,and thirdvelectrodes thereby attenuating said energy in ,itstpassage through thechamber.

2; In a waveguide system, a waveguide section, a combination T-R and attenuating switch gas discharge device providing selective T-R action and attenuation of lowpower radio-frequency energy disposed crosswise of said waveguide section, said device comprising means defining a resonant chamber as anextension of the wall structure of-saidwaveguide section, an ionizable gas in said chamber consisting of 'a mixture of a non-electronegative gas,

and'an electronegative gas, a first electrode disposed in said gas discharge device insulated from said waveguide section for directcurrents and electrically connected to the means defining a resonant chamberat radio frequency and having an axially extended opening therein, a second electrode insulated from. said first electrode and having a source of voltage applied thereto and to said first electrode whereby said fi'rst'and second electrodes serve as keepalive electrodes, means disposing said second electrode coaxiallyin said first electrode, a thirdele'ctrode disposed in axially spaced. relation to said first and second electrodes'and insulated therefrom, a source of voltage, independentiy' operable'switching means to couple said source of voltage to saidfi'rst and "third electrodes to attenuate low-power radio-frequency energy propagating through said chamber, said low-power radio-frequency energy beinginsufiicient tocause'said tube to discharge during the otf periodqof ioperation of said switch, and wherein lllgh-POW5T radio-frequency. energy propagating through;

said chamber will cause an electric discharge within the gas to detune said cavity and prevent passage of the highpower radio-frequency energy therethrough.

3. In a Waveguide system, a waveguide section, a combination T-R and attenuating switch gas discharge device providing selective T-R action and attenuation of lowpower radio-frequency energy disposed crosswise of said waveguide section, said device comprising means defining a resonant chamber as an extension of the wall structure of said waveguide section, an ionizable gas in said chamber consisting of a mixture of a non-electronegative gas and an electronegative gas, an opening in said waveguide section, a sleeve of insulating material disposed within said opening, a tubular first electrode disposed coaxially in said sleeve in said gas discharge device insulated from said Waveguide section for direct currents and electrically connected to the means defining a resonant chamber at radio frequency and having an axially extended opening therein, a second electrode insulated from said first electrode and having a source of voltage applied between said second and said first electrodes whereby said first and second electrodes serve as keep-alive electrodes, insulating means disposing said second electrode coaxially in said first electrode, a third electrode disposed in axially spaced relation to said first and second electrodes and insulated therefrom, said first and third electrodes being of a truncated conical shape having their apexes in juxtaposition thereby forming a capacitance gap, said third electrode only being directly attached to a conductive wall of said chamber and including a mounting having means to axially adjust said third electrode relative to said first electrode to vary the capacitance gap, a pair of oppositely spaced windows in said waveguide section to retain said gaseous medium within said resonant chamber while permitting radio-frequency energy to be passed therethrough, a source of voltage, indipendently operable switching means to couple said source of voltage between said first and third electrodes to attenuate low-power radio-frequency energy propagating through said chamber, said low-power radio-frequency energy being insutficient to cause said tube to discharge during the olf period of said switch, and wherein high-power radio-frequency energy propagating through said chamber will. cause an electric discharge within the gas directly between said first and third electrodes to detune said cavity and prevent passage of the high-power radio-frequency energy therethrough.

References Cited in the file of this patent UNITED STATES PATENTS 2,404,116 Wolowicz et al. July 16, 1946 2,454,761 Barrow et al Nov. 30, 1948 2,491,971 Hall et al Dec. 20, 1949 2,594,732 Cork Apr. 29, 1952 2,625,668 Heins Jan. 13, 1953 2,631,255 Stavro Mar. 10, 1953 2,632,854 Altar Mar. 24, 1953 2,724,791 Carter Nov. 22, 1955 

